November 2016 - kernelnewbies@kernelnewbies.org

where is __percpu_seg initialized?
by 落痕 March 28, 2018

March 28, 2018

Hi all: I knew the kernel implement the per-cpu variable via __percpu_seg. For example on x86 architecture, it's defined as: #ifdef CONFIG_X86_64 #define __percpu_seg gs #define __percpu_mov_op movq #else #define __percpu_seg fs #define __percpu_mov_op movl #endif But I'm curious where __percpu_seg is initialized?? I fail to find the relative code. Thanks.

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How to handle Hotplug with UIO userspace driver
by Mandeep Sandhu Sept. 28, 2017

Sept. 28, 2017

Hi All, We're working on a custom PCIe based hardware, which can be hotplugged into the system. We're using the UIO approach of having a minimal kernel driver which basically handles the interrupt and provides a mmap of the device memory to userspace and userspace driver which mmap's the provided memory and manages the device. We're using systemd and udev rules to spawn off a user process whenever the hardware is plugged into the system. Now my question is, how can we gracefully inform the userspace process if someone pulls out the hardware (sitting on a PCI slot). Even with full hotplug support, there can always be a case of "surprise removal", where someone yanks the board without going through the whole "attention button" routine. While the kernel driver has no problem with hot-removal, I see that if I "unregister" our UIO driver in response to the removal, then the mapping for the hardware's physical memory would become "invalid" (done by UIO on unregsiter). This could possibly cause the user-space process to crash due to illegal memory access. Is there a "standard" way to handle this scenario, i.e for hotpluggable hardware using UIO? Basically, we'd want to "unregister" the UIO device only after the userspace process is informed of the removal so that it could unmap/close the device and exit gracefully. We could try doing it using some systemd trick, where we put a dependency on one of the UIO created files (/dev/uioX), so that when the file is destroyed, systemd could kill/shutdown the process. Although, there's no guarantee that the userspace process won't try to access the mmaped space during that interval, so this isn't a reliable solution either. Any hints, thoughts? Thanks., -mandeep

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What are need to makes humidifier?
by 윤영석 April 1, 2017

April 1, 2017

Hi, I want to make own humidifier. What sensor need when making humidifier?? humidity indicator (detecting humidity), ultrasonic waves maker?? Please shares your wisdom to me. Thanks. Best regards.

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Kernel boot panic
by Amit Mahadik Dec. 12, 2016

Dec. 12, 2016

Hi, I am trying to boot Linux kernel version 4.4.16 on custom ARM based development platform.I want to separate rootfs and kernel Image so that the size of my uImage is small. I followed the following steps. 1. Partitioned the SD card. It has 2 partition. First partition is formatted as vfat partition and has uImage (without rootfs). Second partition is formatted as ext3 and has the rootfs.2. I then changed the U-Boot (version 2011.05) bootargs Kernel command line: console=ttyS0,115200 noinitrd rw rootfstype=ext3 root=/dev/mmcblk0p2 rootwait mem=512M coherent_pool=4M no_console_suspend resume=/dev/mtdblock3. However, while booting I get the following error Failed to create /dev/root: -2VFS: Cannot open root device "mmcblk0p2" or unknown-block(179,2): error -2 I have also attached log for reference. Can anyone please point out what I may be missing? Regards,Amit.

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Fwd: Question about arp caches
by Anupam Kapoor Nov. 30, 2016

Nov. 30, 2016

FYI ---------- Forwarded message --------- From: Anupam Kapoor <anupam.kapoor(a)gmail.com> Date: Tue, 29 Nov 2016 at 1:25 PM Subject: Re: Question about arp caches To: Shawn Peng <yisupeng(a)gmail.com> On Tue, Nov 29, 2016 at 12:49 PM, Shawn Peng <yisupeng(a)gmail.com> wrote: I find a lot references of the function neigh_release(), can you direct me to the one that releases a staled arp entry? neigh_destroy(...) is probably what you are looking for. -- kind regards anupam -- In the beginning was the lambda, and the lambda was with Emacs, and Emacs was the lambda.

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Question about arp caches
by Shawn Peng Nov. 29, 2016

Nov. 29, 2016

Hi, I'm reading the code about arp protocol. It seems like the kernel uses the neigh_table structure to keep track of caches, right? There should be some code that remove the stale entries in the cache, but I didn't find the code that does this job. Does anyone have an idea? Thanks, Yisu

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where does my 4k shared memory go?
by Tien Nguyen Nov. 29, 2016

Nov. 29, 2016

As I read from http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documen… (line 906) *the Shmem field in /proc/meminfo: Total memory used by shared memory > (shmem) and tmpfs* > tmpfs size: /proc # df -Thk |grep -E "tmpfs|Used" Filesystem Type 1K-blocks Used Available Use% Mounted on devtmpfs devtmpfs 1963344 8 1963336 1% /dev tmpfs tmpfs 2027904 728 2027176 1% /dev/shm tmpfs tmpfs 2027904 9936 2017968 1% /run tmpfs tmpfs 2027904 0 2027904 0% /sys/fs/cgroup *total (tmpfs) size: 8+ 728 + 9936 = 10672 kB* shared mem by ipc /proc # ipcs -mu ------ Shared Memory Status -------- segments allocated 23 pages allocated 1855 pages resident 1855 pages swapped 0 Swap performance: 0 attempts 0 successes /proc # getconf PAGESIZE 4096 -->ipcs shared mem = 1855 * 4096/1024 = 7420kB *> 7420kB (ipcs mem) + 10672 (tmpfs) = 18092 Kb* ** /proc # cat /proc/meminfo |grep ShmemShmem: 18096 kB which is different from Shmem in meminfo 4Kb could you help me about "where is 4Kb located?" Thank you so much

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Re: [API] How many time to be context-switched?
by Joel Fernandes Nov. 27, 2016

Nov. 27, 2016

Please reply always to kernelnewbies. On Nov 27, 2016 11:09 AM, "Ricardo Biehl" <rbpoficial(a)gmail.com> wrote: > > 2016-11-27 16:01 GMT-02:00, Joel Fernandes <agnel.joel(a)gmail.com>: > > On Nov 27, 2016 9:44 AM, "Ricardo Biehl" <rbpoficial(a)gmail.com> wrote: > >> > >> As a question I've made on Stackexchange > >> < > > http://stackoverflow.com/questions/40579919/how-many-time-to-be-context-swi… > >>: > >> > >> I'm developing a real-time program where scheduler's context-switches > >> might interfere its performance. > >> --- > >> Suppose I need to do many consecutive (but optional) operations (I > >> mean I'm able to cancel some of them) of the same type in every > >> execution of a timer-oriented signal handler. > >> Could I check how much time left until my thread get context-switched? > >> [...] and then I can cancel/jump some operations for a better > >> performance at all. > >> > >> If yes, how can I do it (in C code)? > >> > > > > If you use SCHED_FIFO, then at the right RT priority level, you should not > > context switch anyway. Time quantas apply only to regular CFS tasks or when > > RT throttling is activated. > > > > Also note that interrupts and softirqs cause variability and interruptions > > no matter what you do or how you schedule. > > > > J. > > Thank you Joel! > > Another small question: Can I use pthread_cond_signal[broadcast]() (a > non async-signal-safe function) inside signal handler, given I know > pthread_cond_wait() hasn't been called at the same time? Should be, as per docs it wakes up *IF* any threads are blocked on cond. Thanks, Joel > > > >> Huge thanks! > >> > >> -- > >> Ricardo Biehl Pasquali > >> > >> _______________________________________________ > >> Kernelnewbies mailing list > >> Kernelnewbies(a)kernelnewbies.org > >> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > > > > -- > Ricardo Biehl Pasquali

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[API] How many time to be context-switched?
by Ricardo Biehl Nov. 27, 2016

Nov. 27, 2016

As a question I've made on Stackexchange <http://stackoverflow.com/questions/40579919/how-many-time-to-be-context-swi…>: I'm developing a real-time program where scheduler's context-switches might interfere its performance. --- Suppose I need to do many consecutive (but optional) operations (I mean I'm able to cancel some of them) of the same type in every execution of a timer-oriented signal handler. Could I check how much time left until my thread get context-switched? [...] and then I can cancel/jump some operations for a better performance at all. If yes, how can I do it (in C code)? Huge thanks! -- Ricardo Biehl Pasquali

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[PATCH 0/7] security: apparmor: apparmorfs.c Checkpatch mods
by Walt Feasel Nov. 26, 2016

Nov. 26, 2016

Make checkpatch modifications to include: Multiple blank lines Blank line after declarations Space after cast WARN_ON vs BUG_ON Align parenthesis Comparison to NULL Line continuations Walt Feasel (7): security: apparmor: apparmorfs.c Multiple blank lines security: apparmor: apparmorfs.c Blank line after declarations security: apparmor: apparmorfs.c Space after cast security: apparmor: apparmorfs.c WARN_ON vs BUG_ON security: apparmor: apparmorfs.c Align parenthesis security: apparmor: apparmorfs.c Comparison to NULL security: apparmor: apparmorfs.c Line continuations security/apparmor/apparmorfs.c | 26 +++++++++++++++----------- 1 file changed, 15 insertions(+), 11 deletions(-) -- Have questions on if the WARN_ON is used properly in patch 4 and also for the comparrison to NULL patch in patch 6 2.1.4

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